1. Field of the Invention
The present invention relates to devices adapted to drive motor vehicle accessories, such as a fan or a water pump or an alternator, from the engine of the vehicle.
The engine of a vehicle runs at a speed which varies within wide limits. It is preferable, however, to maintain those accessories of the vehicle which are driven by said engine, in particular the alternator, at a relatively stable speed. In particular, it is preferable to maintain them at a substantial speed at low engine speeds while avoiding excess speeds at high engine speeds.
2. Description of the Prior Art
It has previously been proposed to provide a driving device for said accessories in which two drive ratios are possible between the driving unit and the driven unit, so that the drive ratio is higher when the driving shaft rotates at a low rate than when it rotates at a high rate.
In particular, such driving devices are known comprising a gear system and control means comprising gears, the system being sensitive to the rotational speed of the driving unit and permitting drive of the driven unit either at an overgeared speed through the gear system, or at the speed of the driving unit through an unidirectional coupling. Such previously proposed devices are relatively complicated and generally require an electro-magnetically driven gear.